Electromagnetic flowmeters can be used to measure the volumetric flow rate of an electrically conductive liquid flowing in a measuring tube. The part of the measuring tube which comes into contact with the liquid is generally electrically nonconductive, so that a voltage induced in the liquid according to Faraday's law of induction by a magnetic field being perpendicular to the axis of the the measuring tube will not be short-circuited.
Therefore, metallic measuring tubes are commonly provided with an electrically nonconductive inner layer and are generally nonferromagnetic; in the case of measuring tubes made completely of plastic or ceramic, particularly alumina ceramic, the electrically nonconduative layer is not necessary.
The induced voltage is picked off by means of at least two galvanic electrodes, i.e., electrodes wetted by the liquid at an end face, or by means of at least two capacitive electrodes, i.e., electrodes mounted, for example, in the wall of the measuring tube. In general, these electrodes are mounted diametrically opposite to each other so that their common diameter is perpendicular to the direction of the magnetic field.
Galvanic electrodes are commonly fitted in a hole in the wall of the flow sensor. They are connected mechanically to the measuring tube in such way that a liquid-tight seal is realized. According to JP-A 4-290 919, particularly FIG. 6, for example, this is achieved by making the diameter of a shank of the electrode smaller than that of the hole and providing the shank with several sealing lips.
However, a problem arises from the fact that the insertion of the electrode results in mechanical stresses in the material in which the hole has been formed or with which it is lined.
In the case of plastic, such stresses result in the plastic "flowing", i.e., the part of the plastic exposed to the stress is displaced toward stress-free areas, particularly toward the interior of the measuring tube, where it forms forward arches which locally reduce the diameter of the measuring tube, thereby influencing the flow of the liquid in an inadmissible manner. This could be reduced by providing the sealing lips only in the part of the hole remote from the liquid.
In the area between the wetted end face and the nearest sealing lip, the prior-art electrode is not perfectly tight, because there a gap exists between the inside wall of the hole and the surface of the electrode shank. The effect of this gap has so far been left out of consideration.
As investigations have shown, the liquid penetrates into this gap more or less far depending on its pressure and/or its temperature and/or its state and/or its type or chemical composition. As a result, the electrode has a wetting surface which is not constant in time, and hence an electric impedance which is not constant in time.
Thus, an additional, temporally nonconstant interference voltage component is superimposed on the electrochemical interference voltage inherent in any galvanic electrode and, therefore, also superimposed on the measuring voltage. This additional interference voltage component is not completely controllable with conventional compensating means as are described, for example, in U.S. Pat. No. 4,382,387 and U.S. Pat. No. 4,422,337.
The electrodes disclosed in GB-A 1 153 295, GB-A 2 047 409, GB-A 2 057 692 and U.S. Pat. No. 4,773,275 try to solve the tightness problem by means comparable to those described in the above JP-A.
In GB-A 1 153 295, an outwardly extending short sleeve forming an electrode pocket is welded to an opening in the metallic measuring tube, and an insulating lining extends into the pocket. The electrode shank is surrounded by a slightly conical insulating bush which is press-fitted in the pocket, so that the end face tries to seal the shank. Thus, the pressure exerted on the lining by the press fit may also cause the aforementioned forward arching, the more so since the point of maximum pressure is located shortly behind the end face.
In the case of the electrode of GB-A 2 047 409, the end face of the electrode has been enlarged to form an electrode head at whose rear side, e.g., the side remote from the lumen of the measuring tube, a ciraumferential claw is provided which forces the material of the liner into the opening, which has a greater diameter than the electrode head. However, this area of the liner, which is subjected to tension, is not sufficiently resistant to thermal shock.
The electrode disclosed in GB-A 2 057 692 also has an enlarged head which draws the liner into the opening for the elecrode shank, so that the resistance to thermal shock in this area is also poor.
Furthermore, the electrode of this GB-A 2 057 692 has a cup-shaped part which is more or less sealingly fitted in a central longitudinal bore of the electrode shank. The outer surface of the bottom of the sleeve is flush with the end face of the electrode, so that a continuous end face is obtained.
According to FIGS. 3, 5, and 8 of this GB-A 2 057 692, however, no importance seems to be attached to the gap between the sleeve and the electrode, since no gap is shown in FIG. 3, while a narrow gap is shown in FIGS. 5 and 8 without being mentioned or explained in the description. In view of the illustrated length and narrowness of this gap it can be assumed that capillary forces act, so that the liquid will penetrate more or less far into the gap depending on the operating conditions and its state. Thus, like in the electrode disclosed in the above-mentioned JP-A 4-290 919, there is no constant wetting surface of the electrode. The electrode disclosed in U.S. Pat. No. 4,773,525 is an electrode of an electromagnetic flowmeter having a ceramic measuring tube. The electrode has an enlarged inner head with an diameter largely greater than that of its shank. The head is tightly fitted to the bottom of a recess in the wall of the measuring tube by using a separate gasket to be pressed between the bottom surface of the recess and the electrode head. The recess has a greater diameter than the electrode head. Therefore, a gap is present between the rim of the head and the wall of the recess.
But no importance seems to be attached to this gap, since it is neither mentioned or explained in the description nor has a reference numeral. Also here, it can be assumed that capillary forces act, so that the liquid will penetrate more or less far into the gap depending on the operating conditions and its state. Thus, like in the electrode disclosed in the above-mentioned JP-A 4-290 919 and GB-A 2 057 692, there is no constant wetting surface of the electrode.